Production of non-aging steel



placement of Patented Aug. 20, 1946 PRODUCTION OF NON-AGING STEEL John D. Gat, Edgewood, and Saylor C. Snyder, Carnegie, Pa, assignors" to Carnegie-Illinois Steel Corporation, a corporation of New Jersey No Drawing. ApplicationJanuary 6, 1943, Serial No. 471,476

9 Claims.

This invention relates to the production of metallic articles, such as steel sheets and strip, which are characterized by a stability of physical properties in the final product usually associated with non-aging steels, and, yet, which are capable of superior performance under conditions of severe deformation, as in deep-drawing operaposition, but must always have a good surface,-

which, depending on the requirements, might be either of mirror-like finish, or of any desired degree of dullness. Furthermore,thissurface qualification must not deteriorate after deep drawing operations are performed.

In some applications, considerable diificulties are liable to develop on account of surface deformation produced by deep drawing operations known as stretcher strains, worms, and many other similar terms. The defect corresponds to a relief pattern on the surface of highly finished sheets. Though this disfiguration does not have any appreciable effect on mechanical quality of the material, it is undesirable, as requiring additional polishing in case of high-grade painting used for automobile bodies, for example, or on account of appearance, when no painting is involved, as in beer cans.

The usual explanation of stretcher strain formation is associated with the phenomena of agehardening. In order than any metallic system can be susceptible to age-hardening, it must consist of metallic base, serving as a solvent, and an element which can dissolve in it at higher temperatures but cannot be held in solution when the temperature drops. At this point the original solid solution decomposes with precipitation of a new phase of some specific composition. This phase, introducing a certain heterogeneity in the metal, causes it to behave in a somewhat different manner than the metal free from it.

Precipitation of phases from solid solution can be accelerated by cold working. When aspeoimen is stressed in a tensile machine, for example, it undergoes elastic deformation up to the point when some of its grains begin to deform plastically. It is generally assumed that the plastic deformation is realized as a relative disintra-crystalline masses along cleavage planes. Strains consequent upon such displacement cause precipitation of a new phase,

the discrete particles of which form at the interfaces of these planes, where they act as keys, retarding or preventing any further relative sliding motion between the definitive masses thereof. As conventionally regarded, this means that the bodies of metal outlined by given sets of planes have become stronger and harder than the neighboring ones, so that further deformation of the total metallic mass under stress must take place along some new sets of planes. Perceptible deformation is associated here with relative displacement of consecutively hardened groups of such bodies. Plastic deformation proceeds, therefore, not in a continuous line, but in steps, made noticeable on highly polished tensile specimens-by a set of lines located at to each other, forming a definite pattern and widely referred to as the above-mentioned stretcher strains. As a practical matter, absence of a solution condition, as by pre-precipitation and, hence, stabilization of dissolved elements, affords smooth performance under deformation, and allows no return of this condition, since there remains no solute to precipitate upon aging.

Many ingenious methods have been proposed already for eliminating or reducing the effect of aging and the degree of attendant stretcher strains formed. Two major lines of approach can be noted here. In one, elements susceptible to precipitation are prevented from doing so by being combined with other elements, forming stable phases insoluble in iron at temperatures found in heat-treating processes applicable to flat steel bodies. Proper addition elements introduced into molten metal achieve this purpose. In the second, phases dissolved in the metal are precipitated by some appropriate heat treatment, usually corresponding to an annealing followed by slow cooling. One of the most successful methods presents. a combination of both practices. In it, steel is treated with a certain amount of degasifiers, and, then, subjected to annealing at a temperature ranging from under the upper critical point to slightly below the lower critical point. Annealing is followed by cooling at a rate regarded as slow, usually in the order of 15 F. per hour.

All of these methods depend on the application of the above principle, but they do not discriminate between individual elements held responsible for precipitation hardening of the steel,

though they pay a particular attention to the agents added for the formation of the new phases. In ignoring the active and deleterious precipitatesparticularly responsible for aging, as

diate size in accordance with the A. S. T. M.

Standard, such, for example, as those correspond- 4 a reasonable speed of strip is maintained, thus, necessitating that continuous methods give way to batch operations, when the cooling of strip is considered.

It is of the essence of the present invention to regard only that element in steel, the precipitatable phases of which are responsible for appearance of the aging phenomenon, and to stabilize substantially such element, either with or without the application of an addition agent to the steel, in conjunction with a heat treatment, selected to afiord optimum results in this respect,

, while, at the same time, avoiding heating and ing to Nos. 3-7 of this scale. Closely associated with this condition, and particularly prevalent in those steels having alloying amounts of aluminum, optimum drawing performance is realized when such intermediate grains are elongated rather than equi-axed. Teachings of physical metallurgy amply demonstrate definite interdependence between time and temperature as a factor in heat treating operations determinative of the ultimate size of the grain. Not only the heat- 'ing controls these features, but the rate of cooling likewise exercises an important influence upon the development of the grain. The methods proposed heretofore for eliminating the precipiitatable phases necessary to free steel sheets from the efiects of aging involve principles whichlead not only toa product of inferior grain characteristics, but, also, which interfere with the applica- .tion of the most efficient processes for this purpose. Thus, as has been already mentioned in this connection, most methods proposedfor freejing steel sheets from the eifects of aging involve, besides occasional use of precipitation elements,

a step of annealing at some selected temperature, 1

followed by slowly cooling. In this connection,

,fannealing corresponds to its meaning common- .ly accepted in the shops, rather than to the full annealing of physical metallur y, in which articles are slowly cooled from above their upper critical temperature. As here used, it embraces the range from just above to just below the lower critical temperature.

Even though higher annealing temperatures,

associated with greater molecular mobility of the metal treated, lead to a faster recrystallization of the metal and aid the avoidance of excessive grain coarsening through critical strain and temperature relationships, still, when followed by the prescribed slow cooling thought necessary for the I complete precipitation and stabilization of the dissolved phases, the resulting grain structure is unduly coarsened and rendered inferior in its drawing properties.

Furthermore, such. slow rates of cooling introduce practically insurmountable difliculties from the engineering standpoint, when the process is intended for use in connection with continuous operations, which are steadily becoming of greater industrial importance. Furnaces, designed for the purpose, efliciently heat the strip to any desired temperature and practically at any selected rate. The same can be said about controlled cooling from such continuous furnaces,

except where the cooling rate is inordinately low, as it has been formerly (and, according-to the teachings hereof, erroneously) held necessary for satisfactory elimination of stretcher strains.

Not even excessively long furnace cooling zones.

afford such a slow cooling rate, particularly when cooling cycles which result in an undesirable condition of the grain.

'It is the primary object of this invention to provide a steel body, which has preferred properties; for severe plastic deformation, as by deepdrawing, and an inherent stability to the end that, once such properties are imparted, they are retained for an indefinite time.

It is a related object to provide a method for the production of such a body, whereby non-aging sheets and strip may be produced efficiently, economically, and in a manner well adapted to modern mill practices and facilities.

Other objects and advantages are implicit in the following description, which, in addition to several specific examples given for purposes of illustrating preferred embodiments of the 'invention, contains theoretical considerations and a general teaching eductive of the spirit and scope thereof, as is apprehended in and by the subjoined claims.

After a long series of experiments, we have discovered that, for practical purposes to be met'in the steel industry, particularly in its deep-drawing branch, only nitrogen enters the picture of age-hardening phenomena, while the rest of the elements, usually associated with aging of steel, can be omitted from consideration practically completely. Furthermore, even the nitrogen content, per se, cannot be treated as directly connected with the aging of steel, unless some quantitative factors are introduced. We have found that nitrogen present in steel in amounts not exjceeding 0.0015% can be considered as innocuous, so that no special treatment need be applied in order to render steel containing it free from aging. On the other hand, larger and moreusual 'amounts of this element must be taken care of preferably by combining a. portion of it with appropriate substances effective to convert it into compounds insoluble at temperatures used in heat treating of sheets, but, in any case, to effect its relative stabilization, whether such compounds are formed or not, as will later appear.

In limiting the aging considerations to the precipitatable phases of nitrogen, drastic reforms in the heating and cooling cycles, previously thought required for complete stabilization, are allowed, since the nitrogen solubility temperature (corresponding to theAa temperature in the iron-carbon system) occurs at or about 700 F. Obviously, this temperature is considerably lower than the sub-critical temperature range in which recrystallization takes place, and

'may be applied for unlimited time to steel bodies without efiecting grain growth. Therefore, as will more fully appear hereinafter, this invention teaches, preferably in conjunction with certain "nitrogen stabilizing additions, the heat treatment of steel. after th application of precise recrystallization annealing temperature for just suchinterval of time as is needed for the attain- '5 lnent of the proper grain size in the metal, below the recrystallization or grain-growth range, but near the nitrogen precipitation temperature, so as to effect complete precipitation and fixation of the nitrogen content, without disturbing the optimum grain condition.

We prepare steel sheets free from grain coarsening and age-hardening tendencies by combining a specific steel making practice with a characteristic heat treating step. In making steel for sheets to which our invention is applicable, no limitations in composition of the metal, nor individual steps of its making in furnaces and subsequent auxiliary equipment, need be considered, with a single exception. Any metallurgical devic suitable for converting components of a metallic charge into finished steel is equally suitable in application to our invention, and any analysis of the metal, which has been found in practice to answer the requirements of deep drawing operations, is acceptable in connection with it. The same holds valid in respect to the metallurgical processes used, all of which are adequate, provided the melting operation is conducted so as to reduce the effective nitrogen content of the finished steel to 0.0015% maximum. Though entirely feasible, the practice leading to the nitrogen content specified above requires exceptional care, and justifies the digression from the standard methods of steel making, constituting on of the steps of a preferred embodiment of the invention, presently to be described.

We propose adding to steel, irrespectively of its belonging either to rimming, semi-killed, or killed types, a sumciency of elements forming relatively stable insoluble compounds with nitrogen to reduce the percentage of iron nitride dissolved in steel to an amount approachin 0.025% Fe4N or 0.015% FezN. Our investigations have shown that titanium and zirconium produce the desired results, aluminum being entirely unsuitable for the purpose, as well as other commonly used degasifiers. The amounts to be added cannot be specified in a general way, being dependent on the degree of bath oxidation and other well- L known physico-chemical relations, but can be easily computed for any individual heat of steel, the state of equilibria of which is, more or less, known; t must be emphasized here that the residual content of the element added for reducing the iron nitride percentage in steel cannot be used, per se, either as an indicator or a measure of the completeness of the desired reaction. This is because standard quantitative analysis technique fails to discriminate between titanium beyond the amount required for nitrogen stabilization, unduly high and wasteful. The apparent residual content of the addition element cannot be taken, therefore, as an indication of complete nitrogen stabilization. Ordinarily, however, such additions would range from one to two and one-half pounds per ton of 17% ferro-titanium applied either in the ladle or in mold, preferably the former. A residue of 0.005% titanium is customarily observed in the ingot analyses.

Ingots of steel so made are then rolled and finished into fiat products following the usual methods familiar to the sheetrnakers, up to the with proper artificial means, to about 700 F., and,

then either held for an appropriate time at this lower temperature or cooled slowly to room temperature. As already explained, the last step effects the precipitation of the nitrogen solute.

In application to the usual box annealing practice, we prefer to stack sheets on bottoms, cover them with intermediate covers, place such assemblages under annealing boxes or covers, and, then, heat the whole in a manner found best in usual sheet-annealing operations. After th desired temperature of the sheet bodies is reached and uniformly distributed throughout the metal, which may be achieved by an appropriate soaking, the annealing bottom, with its lid, is removed from the furnace, and the outside lid removed, leaving intermediate covers in place. Then, any appropriate cooling means, ranging from an air blast to water cooling, is applied until the average temperature of the body of metal reaches around 700 F. At this point, cooling by artificial means is stopped, and the metal is allowed to reach room temperature by radiating its heat into the air with or without any heat insulating cover which might be placed over it at this time. Better results are achieved here when stacks of sheets are subdivided by any suitable means, spacers, for example,.to permit more convenient circulation of the cooling media.

A similar practice is to be followed when coils of strip are to be annealed. Coils are placed on annealing bottoms, protected with an appropriate outside covering, from the action of the atmosphere during the subsequent cooling cycle, placed under annealingcovers, heated to and soaked at temperature, after whichthe bottoms are removed from furnaces, unlidded, and coils cooled by artificial means to about 700 F. average, and allowed to cool slowly to room temperature with or without additional protection of heat-insulating covers. For a more efficient operation, lighter coils are desired in this case.

Our process can be advantageously applied to practices involving continuous annealing. Under conditions found in continuous furnaces, we prefer to raise strip to the desired temperature in the fastest possible manner, cool it, at a rate found to be inducive tothe optimum grain size, by controlling furnace temperatures to about 700 F., and, then, cool the coiled strip slowly to room temperature with or without theuse of means applicable for controlling heat radiation into atmosphere. Continuous furnace operations permit here to increase the cooling rate greatly, as compared with the box-annealing practice. We were able to record entirely satisfactory performance of sheets in deep drawing after they have been cooled to 700 F. at rates approaching 150 F. per minute.

The term slow cooling as applied to our in- Vention corresponds to cooling at a'rate of 20 to 40 degrees Fahrenheit per hour in the interval of 700 to 500 F., cooling from 500 F. toroom .7 temperature can be conducted atany desired rate. In the light of this, the stock, after reaching 500 F., may be-left under a heat insulating cover, or with the latter removed, while artificial cooling may be applied to any desired extent. The selection of the practice to be followed depends, in this case, on economic desiderata.

In this light, the cooling rate required for renclering sheets immune to stretcher strain formation varies between 100 and 150 F. per hour in the temperature interval directly bearing on the elimination of the defect in question, i. e., between the maximum heating temperature and 500 F. Its lowering, by wastefully reducing the cooling rate at which sheets or strip reach room temperaturefrom about 500 F.,ha no ellect upon the real requirements of the successful process, and introduces definite objections into eiiiciency of the whole cycle. I

Increasing the cooling rate cannot be considered here as a mere extension of the temperature range beyond that already known. Cooling not faster than 15 F. per hour was held as a prerequisite for eliminating the recurrence of a definite yield point, the return of the latter being a function of the aging phenomena. Increasing the rate tenfold introduces a new teaching which is in direct opposition to the previous art, and

winch eiiects marked economies, while afiording increased production, and an improved product.

While greatly beneficial in application to any steel intended for deep-drawing purposes, the present invention is particularly advantageous when used in conjunction with steels, the iron nitride contentof which is precipitated and stabilized by the heat-treatment specified, either with or without the use of addition agents, to less than 0.025% FeiN and'to less than 0.015% FezN. Steels so treated, and subjected to the thermal cycle proposed by us, do not show, after the conventional cold pass, a return of the definite'yield point, not only after extended storage, but even after being subjected to an artificial aging treatment of most radical-nature; V

In the accompanying claims,- the term rapidly cooling is intended to mean any cooling rate which, when considered in light of the factors of strain, temperature, and time conducive to the establishment of a. preferred condition of the grain in the steel, will preserve such condition,

and'preclude over-development or coarsening of the crystals beyond the optimum amount. By slow cooling is meant that rate of cooling, including the suspension of cooling in favor of the retention of heat, which will allow satisfactory precipitation of the dissolved iron nitride under conditions approaching equilibrium-solution conditions as the temperature falls to the lower limits of the precipitation range;

We claim:

1. The production of non-aging steel bodies having predetermined structural characteristics which includes: teeming and rolling ingots down to hot-mill finished gauges; cold-reducing the resulting material at leastto within temper-pass of final gauge; annealing the cold-reduced material at temperatures of recrystallization until the desired grain structure isperfected; thereafter, rapidly cooling the material to a temperature at which iron-nitride begins to precipitate; retarding the rapid cooling action within the nitride precipitation range substantially to precipitate all effective amounts of the dissolved nitride, and, then, cooling. at any desired rate to room temperature.

2. In the production of non-aging steel bodies having predetermined structural characteristics that have been reduced substantially to gauge partially by cold-working, and which contain effectiveamounts of nitrogen dissolved therein as iron nitride in the ferrite matrix, the improvement which includes: heat treating such a body at recrystallization temperature to develop a predetermined grain structure; rapidly cooling the body from such temperature down to a. temperature at which iron nitride precipitates; retarding therapid cooling to precipitate substantially all of the effective amounts of iron-nitride remaining in solution, and, then, cooling in any preferred manner to any lower temperature desired.

3. In the production of non-aging steel bodies having predetermined structural characteristics, the improvement which includes: adding to a melt of mild steel a substance capable of reducing substantially the effective amounts of nitrogen by combining with nitrogen dissolved in the metal to form nitrogen compounds insoluble in the solid steel at temperatures of recrystallization thereof, teeming and rolling ingots of such metal to intermediate gauge stock; cold-reducing the hot-worked stock; heat-treating the cold-reduced'stock at a temperature of recrystalhzation; rapidly cooling the stock from such temperature down to the nitride precipitation range; retarding the rapid cooling within this range and cooling at a rate oi substantially 20 F. to 40 F. per hour while above 500 F.

4. In the heat-treatment of non-aging steel stock having predetermined structural characteristics that has been reduced substantially to gauge, the improvement which includes rapidly cooling the stock from temperatures of recrystallization to a temperature at which iron nitride precipitates, and holding the stock for a sufiicient time within the nitride precipitation range to precipitate substantially all effective amounts of dissolved nitride.

5. In the production of non-aging steels having predetermined structural characteristics, the improvernent which includes: reducing the effective amounts'cf nitrogen present in the metal by addition agents capable of forming compounds therewith that are insoluble in the iron at temperatures corresponding to the recrystallization temperatures of the metal; reducing the steel stock to a pro-selected gauge; heat-treating the stock to recrystallize to a predetermined grain structure; rapidly cooling the stock upon attainment of such structure to prevent further grain development; retarding the cooling at or near the iron-nitride precipitation temperature range, and maintaining sufficient temperature substantially to precipitate all eifective amounts of iron nitride remaining dissolved.

6; In the production of non-aging deep-drawi'ng steel bodies having predetermined structural characteristics, the improvement which includes: adding to the steel while molten, a substance capable of forming insoluble compounds with efiective amounts of the nitrogen therein; teeming and reducing the ingots of such steel to gauge; annealing the reduced product at recrystallization temperature to develop grains corresponding to intermediate sizes on the A. S. T. M. scale; arresting the grain development by rapidly cooling the steel to a temperature at which iron nitride dissolved in the metal precipitates, and, then, retaining a sufiiciency of the residual heat for sufficient time to efiect substantially complete precipitation of the remaining dissolved nitrides,

10 temperature, and, then, slowly cooling to at least approximately 500 F.

8. The method of claim 6, in which the insoluble compounds of nitrogen are those formed by at least one of the elements titanium-zirconium.

9. The method of claim '7, in which the insoluble compounds of nitrogen are those formed by at least one of the elements titanium-zir- 10 conium.

JOHN D. GAT. SAYLOR C. SNYDER. 

